This proposal describes a 31/2 year training program for developing a career focused on mechanisms regulating postnatal distal lung morphogenesis. I completed a residency in pediatrics at the University of Colorado Health Sciences Center and a fellowship in pediatric critical care medicine at the University of Alabama at Birmingham. Since accepting a position at Cincinnati Children's Hospital, I have risen from Instructor to Assistant Professor, and I have developed a nascent research program defining the mechanisms of stretch- regulated matrix remodeling in lung development and regeneration. Dr. Jeffrey Whitsett, an accomplished neonatologist and developmental biologist, has a track record of mentoring over 30 successful physician- scientists and has served as my mentor since my arrival. He along with an established scholarship oversight committee will advise me in my transition to a successful, independent research career with R01 funding. The proposed research focuses on the role of stretch-regulated matrix remodeling in septation. Although lung stretch and matrix remodeling are critical factors impacting distal lung morphogenesis, we do not understand how these processes are coordinated or how they regulate alveolar septation. During development and compensatory lung growth stretch induces lung elastin remodeling and regulates fibroblast expression of chymotrypsin-like elastase 1 (Cela1). Cela1 binding to lung matrix increase exponentially with strain, and Cela1 is largely bound to Serpina3 in the extracellular matrix. This proposal tests the overall hypothesis that stretch-regulation of Cela1 activity is critical to alvelar septation. Aim 1 of this proposal tests whether Serpina3 is the putative inhibitor of Cela1 in the lung matrix by pull-down and mutation of the Serpina3 reactive center loop. To define the stretch-dependent mechanism by which Cela1 binds lung tropoelastin, Aim 2 measures the extent to which specific tropoelastin peptides quench the binding of Cela1 to lung matrix with increasing strain. It then tests the critical Cela1 amino acids mediating this interaction by site directed mutagenesis. Aim 3 first defines the lung phenotype of the Cela1 knockout mouse and quantifies Cela1's contribution to lung elastase activity. Subsequently, it tests the importance of Cela1 in the physiologic matrix remodeling of regeneration and the pathologic matrix remodeling of emphysema. The career development portion of this proposal is foundational for my transition to research independence. It provides training in bio-mechanical model generation and testing. When combined with dynamic imaging of ex vivo lung sections and methods for quantifying different cellular and matrix-remodeling processes, this knowledge will permit mechanistic insight into how stretch and strain regulate distal lung morphogenesis. On a molecular level, this proposal also will provide training in molecular model generation and the methods testing these models. This training and the associated collaborative relationships will be key in developing a multidisciplinary, transformative research program focused on improving lung health in children with BPD.